This relates generally to imaging devices, and more particularly, to imaging devices with multi-photodiode image pixels controlled using vertical transfer gates.
Image sensors are commonly used in electronic devices such as cellular telephones, cameras, and computers to capture images. In a typical arrangement, an electronic device is provided with an array of image pixels arranged in pixel rows and pixel columns.
Conventional image sensors include photodiodes formed using dopant implantation. Image pixels are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology. The image sensors may include photodiodes and other operational circuitry such as transistors formed in a front surface of the substrate. A single image sensing pixel in the two-dimensional array of conventional image sensing pixels includes a single photosensitive region, a color filter formed over the photosensitive region, and a single dome-shaped micro-lens formed over the color filter.
There is often undesired electrical cross-talk between adjacent pixels associated with different colors. Undesired electrical cross-talk is characterized by photogenerated charges generated in the semiconductor region of a pixel associated with one color being collected by the photosensitive region (i.e., the photodiode) of a pixel associated with a different color. An example of undesired electrical cross-talk is when photogenerated charges that were generated in response to red light diffuse into and are collected by a photosensitive region associated with a green pixel (i.e., a photosensitive region that should receive green light and generate charges corresponding to the amount of green light received). Electrical cross-talk can also degrade the output image quality of an imager.
Moreover, in conventional imaging systems, image artifacts may be caused by moving objects, a moving or shaking camera, flickering lighting, and objects with changing illumination in an image frame. Such artifacts may include, for example, missing parts of an object, edge color artifacts, and object distortion. Examples of objects with changing illumination include light-emitting diode (LED) traffic signs (which can flicker several hundred times per second) and LED brake lights or headlights of modern cars.
Conventional imaging systems also can have images with artifacts associated with low dynamic range. Scenes with bright and dark portions may produce artifacts in conventional image sensors, as portions of the image may be over exposed or under exposed. High dynamic range operation of pixels without the introduction of additional motion artifacts is an important requirement in many applications. An example of an application in which both high dynamic range operation of an image sensor and artifact-free images are critical is the automotive industry, where image sensors may be used at least in part to determine the course and/or operation of a motor vehicle.
It is therefore desirable to be able to provide improved image pixels and image sensors for imaging devices.